The invention is based on a generic hydraulic unit for a vehicle brake system. From European Patent Disclosure EP 0 621 836 B1, one such generic unit is known that is embodied as immersion-proof. The hydraulic unit has at least one pump that has a piston, and the piston is indirectly drivable by an electric motor via an eccentric that is rotatable inside an eccentric chamber. Because it cannot be precluded that the moving piston may entrain pressure fluid into the eccentric chamber through a sealing ring or the like on the occasion of pump operation, a conduit extends downward from the eccentric chamber and opens to the atmosphere through a housing block that receives the at least one pump; a check valve that can open toward the atmosphere is built into the conduit. Pressure fluid that has been entrained into the eccentric chamber and drips from the piston enters the conduit and stresses the check valve, causing it to open, and finally flows out of the housing block. As a result, filling of the eccentric chamber with pressure fluid is averted, so that pressure fluid remains far away from the electric motor, or in other words does not penetrate the electric motor and impair the operability thereof.
In a hydraulic unit known from International Patent Disclosure WO 96/13416, the electric motor is located above an eccentric chamber, in which the eccentric acting on at least one piston of a pump is rotatable. Once again, a conduit leads downward from the eccentric chamber, directly or indirectly to a so-called central venting point, where an element is installed that is permeable to air but not to water from the environment, so as to equalize the pressure of the eccentric chamber, for instance, relative to the ambient atmosphere and on the other hand to prevent at least splashing water from entering the unit. As an additional provision, the central venting point may have a check valve mentioned in this aforementioned patent disclosure. For instance, a check valve disclosed in the first reference cited, EP 0 629 836 B1, could be used. The unit also has at least one reservoir intended for temporarily receiving pressure fluid; it comprises a cylindrical hollow chamber closed off from the ambient atmosphere, a piston displaceable in the chamber that divides the hollow chamber into a variable storage chamber and a spring chamber, and a spring acting on the piston. The spring is braced on a cap that tightly closes off the hollow chamber. Extending from the spring chamber, adjacent to the cap, a pressure equalization conduit, which leads either upward to an inner hollow chamber in the electric motor or downward to a hollow chamber in a covering hood. The covering hood covers electromagnets of valves of the unit, as well as electrical components that are disposed on a circuit board. In operation of the unit, pressure fluid entrained into the eccentric chamber by a piston of the pump drips directly into the covering hood, for instance, so that it cannot be precluded that electrically operative components will become moistened with the pressure fluid.